1. Field of the Invention
The present invention relates generally to machine tools such as punch presses and laser processing machines and more particularly to clamping apparatus for clamping materials to be processed such as sheet materials in machine tools such as punch presses, laser working machines and drilling machines.
2. Description of the Prior Art
The prior art concerning the present invention will be described, by way of example, by reference to what is called a turret punch press. A turret punch press has turrets for holding punching tools to make holes in sheet materials such as metal sheets. The present invention is applicable not only to other punch presses in addition to turret punch presses, but also to other machine tools such as laser processing machines.
As is well known, the turret punch press comprises a vertically movable ram and a pair of rotatable upper and lower turrets for holding a plurality of upper and lower punching tools which are varied in size and shape to punch a variety of holes in sheet materials such as metal sheets. The upper and lower turrets are substantially vertically spaced from each other beneath the ram and are horizontally disposed on their respective shafts, which shafts are vertically disposed so as to coaxially align with each other. Each of the upper punching tools on the uppr turret is so located as to vertically align with either of the lower punching tools on the lower turret to cooperate with each other to punch holes of a peculiar shape. Also, the upper and lower turrets are so arranged so to be simultaneously rotated by power to bring a desired pair of the upper and lower tols into a portion just beneath the ram so as to enable them to be worked by the ram to punch holes of a desired shape. In this arrangement, a workpiece such as a metal sheet is horizontally fed by a plurality (usually a pair) of clamping means into a space between the upper and lower turrets; namely the upper and lower punching tools which have been placed just beneath the ram by the upper and lower turrets. The clamping means are so arranged as to grip an end of the workpiece and to move the workpiece in any direction toward and away from the upper and lower turrets in order to bring any portion of the workpiece into a position beneath the ram. Also, in order to automatically and continuously punch a number of holes varying in size and shape in the workpiece, the upper and lower turrets and the clamping means are so arranged as to be rotated and moved under a preprogrammed numerical control.
In the conventional turret punch press of the above described arrangement, it has been disadvantageous that the clamping means for clamping the workpiece will often inevitably go into collision with any of the lower punching tools on the lower turret. The clamping means will collide with any of the lower punching tools when it is moving in the proximity of the upper and lower turrets to place just beneath the ram a portion of the workpiece close to the clamped end thereof. Since the clamping means is moved by power at high speed, the clamping means and the lower punching tools will suffer from great shock which will shorten their useful lives and decrease the punching accuracy, when they collide with each other. Accordingly, it has been necessary to decrease the shock which is caused by the collision of the clamping means and lower punching tools, especially in order to move the clamping means at high speed to perform punching operations at higher speed.
As another conventional disadvantage with regard to the turret punch press, workpieces such as metal sheets are often originally bent or warped and also will become bent or warped upwardly away from the horizontal level when continuously punched. Accordingly, there has been a need that the clamping means cope with the bends or warps of workpieces being punched in order to perform accurate punching operations without damaging the workpieces.